This invention relates to fuel cells, and particularly to fuel cell power plants suited or intended for use in transportation vehicles, or as portable or stationary power plants. More particularly still, the invention relates to a method and apparatus for preventing water in fuel cell power plants, and particularly proton exchange membrane (PEM) type fuel cell power plants, from freezing during periods of inactivity and storage.
Fuel cell power plants are commonly used to produce electrical energy from oxidizing and reducing fluids, such as oxygen, or air, and hydrogen, respectively. The electrical energy may be used to power electrical apparatus in a variety of environments, including in space vehicles, in land-based vehicles, and/or in a variety of other stationary and mobile applications. In such power plants, a plurality of planar fuel cells are typically arranged in a stack which receives and/or provides flows of a reducing reactant, such as hydrogen, an oxidant reactant, such as oxygen or air, coolant and product fluids. Each individual cell generally includes an anode electrode receiving the hydrogen reactant, a cathode electrode receiving the oxidant reactant, and an electrolyte, such as a proton exchange membrane (PEM), between the anode and cathode electrodes. Each cell typically also includes associated structure for the introduction, flow through and/or removal of coolant and product fluids, such as water.
While having important advantages, fuel cells and particularly PEM cells, also have limitations related to liquid water transport to, through, and away from the cell. Use of such fuel cells to power a transportation vehicle or other apparatus in a cold environment gives rise to additional concerns associated with water management, such as preventing mechanical damage occasioned by the freezing of the product water and/or any water coolant fluid, and minimizing the inconvenience of re-starting delays in the event of such freezing of product water and/or water coolant fluid. For applications in which a fuel cell power plant powers a vehicle, there is a general requirement that they be capable of startup and drive away in subfreezing ambient conditions as severe as xe2x88x9240xc2x0 C. within 10 seconds, and no permanent damage from a hard freeze to xe2x88x9250xc2x0 C. The startup condition cannot be met if ice forms during storage, which must be thawed prior to boot-strap starting using only internal power.
One approach to providing a freeze tolerant fuel cell power plant is described in U.S. patent application Ser. No. 09/935,254 filed Aug. 22, 2001 for xe2x80x9cFreeze Tolerant Power Plantxe2x80x9d, which application is assigned to the assignee of the present application and is incorporated herein by reference. In that application, a water displacement system having a freeze tolerant accumulator that contains a water immiscible fluid and water coolant is employed for removing water in cooling channels at shutdown. Some provision is made for preventing freezing of water coolant for short periods of shutdown by supplemental heating of the water immiscible fluid. However, for shutdowns for an extended period, i. e., xe2x80x9cstoragexe2x80x9d of more than several days during subfreezing weather, the water content in the accumulator portion of the system freezes and requires excessive time and energy to be melted for startup.
Another approach to the maintenance of a suitable operating temperature in the cell stack assembly during periods of cold ambient temperatures, at least during operation, brief shutdown, and restart, is described in PCT International Application PCT/CA00/01500, published Jul. 5, 2001 with Publication Number WO 01/48846 A1, entitled Method and Apparatus for Increasing the Temperature of a Fuel Cell Stack. That application describes combusting fuel reactant and oxidant reactant within either the coolant flow pathway or a reactant flow pathway to heat the stack assembly to a desired temperature during operation, brief shutdowns and/or for restarts. Keep warm methods that include stack components may be desirable in some circumstances, but generally require more complex, and therefore costly, control schemes. The more stringent requirements are needed to protect stack components from excessive temperatures or other extreme conditions that could cause irreparable damage. Because of their complexity, such approaches would also be more energy demanding and therefore would require greater fuel consumption which limits the storage protection time available.
Even though the energy required to melt the amount of ice expected from a hard freeze can be obtained from stored fuel reactant, such as H2, the power needed to melt it so positive power can be generated within 10 seconds exceeds the power rating of the power plant itself. If the fuel reactant is to be used directly for heat by combustion, the heat needed for such rapid melting could damage the system and would be a serious drain on the fuel supply.
Accordingly, it is an object of the invention to provide an arrangement that will enable a fuel cell power plant to generate power rapidly, even following shutdown storage for relatively long intervals under very cold conditions.
It is a further object of the invention to afford the aforementioned capability using the fuel cell fuel source.
It is a still further object of the invention to afford the aforementioned capability in a fuel-efficient manner.
The present invention provides a keep-warm system for a fuel cell power plant. The fuel cell power plant may include a PEM-type fuel cell stack assembly (CSA) having anode(s), cathode(s), proton exchange membrane(s), and a cooler, typically a water transport plate(s). However, the keep warm system can apply to any type of fuel cell power plant that contains components and/or fluids that are subject to freezing at temperatures in the xe2x88x9250xc2x0 C. range. The power plant further includes means, such as a storage tank of hydrogen, for supplying a hydrogen-rich fuel, such as hydrogen, at least to the anode, and a source of oxidant reactant, such as air, for supplying the cathode. A water management system is included with the power plant. In accordance with the keep-warm system of the invention, there is also provided one or more thermal insulating enclosures for the power plant, including the CSA and the water management system, as well as a catalytic burner for convectively supplying heat to the insulating enclosures and the power plant components therein. The stored hydrogen is selectively used to fuel the catalytic burner. The hydrogen is fed to the catalytic burner where it mixes with a supply of air and contacts a catalytic surface of the burner to effect a flameless oxidation reaction that releases heat at a moderate temperature, typically in the range of 200xc2x0-700xc2x0 F. The heat is contained in the combusted gas and is carried by convection, into and through the insulating enclosures to exchange heat and warm the freeze-sensitive components of the power plant contained therein. That convective heat is the principal source of keep warm heating.
The rate of flow of hydrogen fuel and air to the catalytic burner need not be large, and is readily provided by selective feed of the pre-pressurized hydrogen from storage and the associated induction of ambient external air resulting from the convective flow of the heated gas. The flow of hydrogen to the burner, and thus, at least in part, the resultant heat provided, is governed by regulating the flow rate and/or flow intervals as a function of temperature, typically sensed at or near the freeze-sensitive components requiring protection from freezing. That temperature threshold, or control temperature, is typically about 5xc2x0 C. (40xc2x0-45xc2x0 F.)
The CSA and the water management system may be located in a common thermal insulating enclosure and may be arranged for optimal utilization of heat contained in the convectively conveyed gases which pass thereby, or there through, in heat exchange relation. Alternatively, there may be multiple insulating enclosures each containing different parts of the power plant, and appropriate interconnection passages for flow of heated gas there between. There may also be various heat exchangers for the heated gas, to the extent required. Appropriate venting for cooled exhaust gas and drainage for condensation from that cooling, are provided.
The forgoing arrangement of pressurized hydrogen, induced air, catalytic burner, convective heat flow, and insulated enclosure(s), provides sufficient heat to the power plant to keep it from freezing for an extended xe2x80x9cstoragexe2x80x9d period. Using only stored hydrogen and substantially no electrical power to drive parasitic loads, such as pumps and/or blowers, that storage period may be 7 days or more, depending upon the external temperatures and the supply of hydrogen available. While the power plant is described as being a hydrogen-fueled unit, the keep warm approach can also with other types of gaseous or light liquid fuels such as gasoline. In the case of gasoline, the burner system would require the addition of a fuel vaporizer in order to vaporize the gas before it reaches the catalytic burner.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.